The standard mode of precise differential positioning uses one reference receiver located at a station whose coordinates are known, while determining a second receiver's coordinates relative to the reference receiver. In addition, the second receiver may be static or moving, and carrier phase measurements must be used to assure high positioning accuracy. This is the basis for pseudo-range-based differential global positioning system (GPS) (DGPS for short) techniques. However, for high precision applications, the use of carrier phase data comes at a cost in terms of overall system complexity because the measurements are ambiguous, requiring that ambiguity resolution (AR) algorithms be incorporated as an integral part of the data processing software.
Such high accuracy techniques result from progressive research and development (R&D) innovations, subsequently implemented by GPS manufacturers in top-of-the-line “GPS surveying” products. Over the last decade, several significant developments have resulted in the high accuracy performance also being available in “real-time”—that is, in the field, immediately following measurement, and after the data from the reference receiver has been received by the (second) field receiver for processing via a data communication link (e.g. very high frequency (VHF) or ultra high frequency (UHF) radio, cellular telephone, frequency modulation (FM) radio sub-carrier or satellite communication link). Real-time precise positioning is even possible when the GPS receiver is in motion through the use of “on-the-fly” (OTF) AR algorithms. These systems are commonly referred to as “real-time kinematic” (RTK) systems, and make feasible the use of GPS-RTK for many time-critical applications, e.g. machine control, GPS-guided earthworks/excavations, automated haul truck operations, and other autonomous robotic navigation applications.
If the GPS signals were continuously tracked (loss-of-lock never occurred), the integer ambiguities resolved at the beginning of a survey would be valid for the whole GPS kinematic positioning span. However, the GPS satellite signals are occasionally shaded (e.g. due to buildings in “urban canyon” environments), or momentarily blocked (e.g. when the receiver passes under a bridge or through a tunnel), and in these circumstances the integer ambiguity values are “lost” and must be re-determined or re-initialized. This process can take from a few tens of seconds up to several minutes with present OTF AR techniques. During this “re-initialization” period, the GPS carrier-range data cannot be obtained and there is “dead” time until sufficient data has been collected to resolve the ambiguities. If GPS signal interruptions occur repeatedly, ambiguity re-initialization is, at the very least, an irritation, and, at worst, a significant weakness of commercial GPS-RTK positioning systems (see upper plot in FIG. 1). In addition, the longer the period of tracking required to ensure reliable OTF AR, the greater the risk that cycle slips occur during the crucial (re-)initialization period. A loss of lock of a receiver phase lock loop causing a sudden integer number of cycles jump in a carrier phase observable is known as a cycle slip. Receiver tracking problems or an interrupted ability of the antenna to receive satellite signals causes the loss of lock. These shortcomings are also present in any system based on data post-processing as well.
A goal of all GPS manufacturers is to develop a real-time precise GPS positioning system, able to deliver positioning results on demand, in as easy and transparent a manner as is presently the case using pseudo-range-based DGPS techniques, which typically deliver positioning accuracies ranging from 1 to 10 meters. The ambiguity initialization period must be kept as short as possible, or even to the extreme case “instant”. Three general classes of AR techniques have been developed in the last decade: search techniques in the measurement domain; search techniques in the coordinate domain, and; search techniques in the estimated ambiguity domain using least squares estimation. In general, AR OTF using these techniques requires several epochs of data, causing a time delay for real-time applications. An integrated technique was then developed to take advantage of most positive characteristics from all three general classes of AR techniques, such as search efficiency or reliability, and hence make instantaneous AR more certain (Han & Rizos, “Integrated Method for Instantaneous Ambiguity Resolution Using New Generation GPS Receivers,” IEEE PLANS '96, (April, 1996), pp. 254–261. However, due to the smaller degrees-of-freedom in comparison to AR OTF, quality control is a very important issue. A three-step quality control procedure was further developed to solve these problems (Han, “Quality Control Issues Relating to Ambiguity Resolution for Real-Time GPS Kinematic Positioning,” Journal of Geodesy, Official Journal of the International Association of Geodesy (1997) 71(6), pp. 351–361.
There is a need in the art for an improved method of processing GPS signals to rapidly achieve high accuracy, reliable position determination. Further, there is a need in the art to apply quality control mechanisms to improve the position determination method.